What is CFRP?
CFRP (Carbon Fiber Reinforced Plastic) is an advanced light weight composite material made up of carbon fiber and thermosetting resins.
Machining Carbon Fiber for Post Processing
Machining carbon fiber – post processing is the final phase and once complete, the CFRP part is ready to be put into assembly. In post processing, carbon fiber trimming removes excess material if needed and cutting carbon fiber is used to machine part features into CFRP. Using a robotic waterjet or robotic router- unrivaled accuracy and speed using robotics for CFRP post process trimming, and laser software and router software technology can make all the difference.
Robotic carbon fiber trimming systems are easy to use, easy to maintain and easy to recover. Learning Path Control (LPC), and Learning Vibration Control (LVC) combined with Adaptive Process Control (APC) technologies supercharge the speed of the robotic trimming up to 60% beyond what is possible out of the box. Accufind and iRCalibration are technologies that use IR and CCD vision technology to keep pinpoint path accuracy while maintaining high speed cutting of the CFRP.
Waterjet, dry router and wet router technologies can all be suitable for carbon fiber trimming or cutting carbon fiber depending on the properties of the part and the production requirements. A variety of studies and tests are available to find the most optimal carbon fiber cutting solution for the specific CFRP part.
The Fiber in CFRP
CFRP starts as an acrylonitrile plastic powder which gets mixed with another plastic, like methyl acrylate or methyl methacrylate. Then, it is combined with a catalyst in a conventional suspension or solution polymerization reaction to form a polyacrylonitrile plastic.
The plastic is then spun into fibers using one of several different methods. In some methods, the plastic is mixed with certain chemicals and pumped through tiny jets into a chemical bath or quench chamber where the plastic coagulates and solidifies into fibers. This is similar to the process used to form polyacrylic textile fibers. In other methods, the plastic mixture is heated and pumped through tiny jets into a chamber where the solvents evaporate leaving a solid fiber. The spinning step is important because the internal atomic structure of the fiber is formed during this process.
Then the fibers are washed and stretched to the desired fiber diameter. The stretching helps align the molecules within the fiber and provide the basis for the formation of the tightly bonded carbon crystals after carbonization. Before the fibers can be carbonized they must be chemically altered to change their linear atomic bonding to more stable ladder bonding. To do this, the fibers need to be heated in air to around 380-600 F for an hour or so. This makes the fibers pick up oxygen molecules and rearrange the atomic bonding structure. Once this process is complete the fibers will be stabilized.
Once the fibers are stable, the carbonization process begins. The fibers are heated to 1800F to 5300F for a few minutes in a furnace filled with a gas mixture and no oxygen. A lack of oxygen prevents the fibers from catching fire at the high temperatures required for this step. The oxygen is kept out by an air seal where the fibers enter and exit the furnace and keeping the gas pressure inside the furnace higher than the outside air pressure. While the fibers are heated they start to lose their non-carbon atoms in the forms of gasses like water vapor, ammonia, hydrogen, carbon dioxide, nitrogen and carbon monoxide.
As the non-carbon atoms are removed, the remaining carbon atoms start to form tightly bonded carbon crystals that align parallel to the long side of the fiber. After this carbonization process is finished, the fibers will possess a surface that does not bond well. In order to give the fibers better bonding properties their surface needs to be oxidized, giving the fibers a rough texture and increasing their mechanical bonding ability.
Next is the sizing process. For this the fibers are coated with a material such as epoxy or urethane. This protects the fibers from damage in the winding and weaving phase. Once the fibers are coated they’re spun into cylinders called bobbins. The bobbins are then put in a machine that twists the fibers into yarns. Those yarns can then be used to weave a carbon fiber filament fabric.
In the next step a lightweight, strong durable skin is created using a process called overlay. In this process carbon fiber fabric is laid over a mold and combined with resin to create its final shape. There are two methods that can be used to for the overlay process. The first is called “wet carbon fiber layup”. For this process a dry carbon fiber sheet is laid over the mold and wet resin is applied to it. The resin gives the carbon fiber stiffness and acts as a bonding agent. The second process is called “pre-preg carbon fiber lay up”. This process uses fiber that is impregnated with resign. Pre-preg lay up provides much more uniform resin thickness than the wet lay up method due to superior resin penetration in the carbon fiber. There’s also Resin Transfer Molding (RTM)- which takes place in the next step but combines the molding step and preform carbon fiber resin transfer step into one process; more on RTM below.
Now that the CFRP prepared for forming, it’s time to mold it into a permanent shape. There are variety of techniques that can be used for the molding process. The most popular is compression molding. Compression molding involves two metal dies mounted in a hydraulic molding press. The CFRP material is taken out of the lay up and placed into the molding press. The dies are then heated and closed on the CFRP and up to 2000psi of pressure is applied. Cycle time can vary depending on part size and thickness.
Recent breakthroughs such as BMW’s “wet compression molding” process have dramatically decreased compression mold cycle time. Resin transfer molding or “RTM” is another commonly used molding technique. Like compression molding, it features dies mounted in a press that close on the preform CFRP. Unlike compression molding, resin and catalyst are pumped into the closed mold during the molding process through injection ports in the die. Both the mold and resin may be heated during RTM depending on the specific application. RTM can be preferable to other molding methods because it reduces the steps to create CFRP by combining some of the tradition preform phase steps into the molding phase.